I came across the following sentence
ところで、リー君はどうして忙しいの?
translated as
By the way, Lee-kun, why are you busy?
If the の is left out, what's the difference in 'feel' or implication? That is, why is the sentence above different from
ところで、リー君はどうして忙しい?
From what I've read, の seeks more explanation and context, but for a 'why' question, is an explanation not already expected? What purpose does the の play in the first sentence?